The tooth fairy could not have been kinder. A set of four teeth in a tiny jaw discovered early this year on a beach in southern Australia have turned out to be the oldest mammal fossil yet found in Australia. And paleontologist Thomas Rich of the Museum of Victoria in Melbourne, Australia, who describes[2] the fossil in tomorrow's Science, thinks it may be a most un-Australian mammal: a placental mammal--one that nourishes its developing embryo within the mother's uterus. That would put placental mammals down under 110 million years earlier than believed, upending paleontologists' ideas about mammal evolution.

Most paleontologists think higher mammals--placentals and pouched marsupials--diverged from a common ancestor in the Early Cretaceous period, between 144 million and 98 million years ago. And based on the smattering of known Cretaceous fossils, they believe placental mammals originated in Asia, then migrated to North America. Marsupials reached Australia early, but terrestrial placentals are not thought to have entered Australia until about 5 million years ago.

But Rich thinks the fossil teeth and jaw have features that link the shrew-sized creature, Ausktribosphenos nyktos, to placental mammals. If the identification holds, says Richard Cifelli, curator of vertebrate paleontology at the Oklahoma Museum of Natural History in Norman, "all bets are off" as to where placentals originated. "If they were in Australia then," he says, "there's no reason they couldn't have also been in South America, Antarctica, and perhaps Africa. You could make an argument for any place of origin."

But other researchers say that the evidence is ambiguous, noting that the jaw has an odd mixture of primitive and advanced traits. That could suggest that the fossil is not a placental at all, but a remarkable new animal that should spark new thinking about mammal evolution on the southern continents. Says Cifelli: "It's extremely important whatever it is."